Question: what is ? The “obvious” answer is to rewrite and then just substitute. The mathematical answer is to note that we do get a limit as we approach the point from any point in where is a deleted disk and is the domain of the function, which of course, does not include the line . However, most calculus books just speak of the deleted disk without mentioning the domain; the interesting thing here is that the point is NOT an isolated (if inessential) singularity.
This is a case in which being too precise might hinder understanding, but not being precise enough can lead to confusion.